Last week’s Facebook announcement was not much of a surprise. Everyone knew Zuck & Co. were out to invade Android, offering big, blue and white thumbs to stamp on every aspect of a user’s mobile experience. Sure, Chatheads are neat, and coverflow looks pretty. But Facebook Home, in and of itself, isn’t that big of a deal. What it represents, however, is huge. We’re calling Home an apperating system, one of a new breed of software platforms that sit between operating systems and apps. Apperatingsystems are coming—in a major way.

Facebook Home is the most fully realized apperating system yet, enveloping the underlying Google Android operating system with a radical new look and augmenting it with important and genuinely useful new social features, all while preserving perfect compatibility with Android apps, including anything you might download through the bundled Google Play app store. It proves that a mobile platform can be custom tailored for a particular audience or context without sacrificing power and extensibility.

But Facebook Home is hardly the first of its class. Amazon’s Kindle Fire tablet contains probably the most prominent apperating system, a suite of apps and Android customizations developed by Amazon engineers. Software makers have also developed platform layers on top of Apple’s iOS, a system that offers far fewer opportunities for integration than Google’s famously customizable Android. As apperating systems spread and improve, they will help Android and iOS better serve niche audiences and serve as labs for features that migrate back to the host system and into general use. At the same time, they’ll raise thorny questions about the appropriate balance of power between operating system vendors like Google and Apple on the one hand and app makers like Facebook and Amazon on the other.

A slide from the Facebook Home launch event shows how Facebook Home sits between the Android operating system and apps — i.e., it’s an apperating system. Photo: Alex Washburn/Wired

More so than Facebook Home, the Kindle Fire already seems to be pushing the limits of the operating system/apperating system relationship. The Fire ejects Google’s digital store, Google’s browser, and Google’s email client from Google’s own operating system, replacing them with Amazon-native alternatives. Unlike with Facebook Home, installing core Google services like the Google Play app store and basic Android apps involves hacking the device and voiding your warranty.

Still, Facebook Home makes its own big changes to the default Android experience. Most significantly, it buries most Android apps several clicks away from the home screen, meaning they are less likely to be used — or even discovered — by consumers. Facebook content and advertisements, meanwhile, will get top billing, appearing even when a user has the phone or tablet locked.

These system tweaks could mean a big loss of revenue for Google. The company spends heavily to develop Android but freely shares the source code, betting that the operating system will usually be distributed in the default, Google-approved configuration, in which it is bundled with Google’s suite of proprietary apps like Gmail, Chrome, and Google Maps. These apps, rather than Android per se, are where the money comes from; once in use, they begin displaying advertisements funneled into the device by Google, ads that can be carefully targeted based on factors like location.

Undermining the link between Android and Google’s proprietary apps, as Facebook Home and Kindle Fire do, upsets the Android business model.

Apple’s tightly-controlled iOS faces its own subversives. A proprietary, locked-down system, iOS isn’t nearly as customizable as Android. But Apple’s restrictions have only fueled the rise of Dropbox, a third-party system that allows data to be more easily shared between apps, silo-ed off from one another within iOS, via Dropbox servers. Originally designed to sync files between desktop computers, Dropbox has become “the linchpin in the workflow” for Apple’s iPad, in the words of Apple fan John Gruber, thanks to the fact that a great many iPad productivity apps have added Dropbox sharing hooks.

Normally, apps on iOS are locked into their own sandboxed portion of a device’s local flash storage, meaning they can’t read files written by other apps or by the same app on a different device. If you have a Dropbox account, however, apps can send data over the internet to your global Dropbox folder, assuming the apps are Dropbox enabled, and read data from that same folder. If you install the Dropbox app, you can even use it to send files to apps that aren’t Dropbox enabled, like Apple’s Pages.

Also on iOS, Google has launched its own constellation of apps that share data with one another, including Google+, Snapseed, Gmail, Google Maps, Google Talk, and Google Voice. Many, like Gmail and Google Maps, are arguably the best in their class on iOS, and together they form a sort of apperating system.

Apperating systems can find themselves in conflict with the operating systems on which they are based. If they add a useful capability, as Dropbox does, the operating system might try and co-opt their functionality, as Apple’s own iCloud has tried (poorly) to do with Dropbox. If they undermine the goals of the operating system vendor, meanwhile, they might find themselves banned, as when Apple tried to block certain Google apps from its iOS store (federal regulators eventually intervened)

Because of this inherent tension, Facebook executives faced a barrage of questions this week over whether Google would find a way to lock Home out of Android in future versions.

“Anything can change in the future, but we think Google takes their commitment to openness in this ecosystem really seriously,” Facebook CEO Mark Zuckerberg told assembled members of the press. “Their operating system really is designed from the ground up to support these things. It is theoretically possible that they go back on their commitment to openness, but I don’t think they will. It would take a lot of really concerted effort to change the rules of something like this and make the system different… It would be a complete 180 on their philosophy and promise of openness.”

Of course, revenue threats have a way, historically, of undermining prior commitments in the tech world. The best way for apperating systems to protect themselves isn’t to collect promises but to make themselves indispensable to users. Dropbox is well on its way; whether Facebook Home and Kindle Fire ever become quite so essential remains to be seen.